The fortress dominating the entrance to the Venetian harbour has been variously referred to as the Castello del Molo, the Rocca a Mare and the Koules.

It predates the second Venetian fortifications around the city, and was built so as to protect the area enclosed by the mole. The Venetian Castello acquired its final form over the period 1523-1540, in replacement of an older fortress destroyed by earthquakes and other phenomena.

The fortress is first attested in the early 13th century the earliest depiction we have is that by the Florentine monk Cristoforo Buondelmonti in 1429.

The devastating force of the sea was such that the walls and foundations were in constant need of repair.